Helicopter rotor assemblies, and in particular the rotor blades thereof, are subjected to a variety of operational forces--aerodynamic, inertial, and centrifugal. In particular, rotor blades must be designed to accommodate various dynamic loads such as bending loads, both flapwise (out-of-plane) and chordwise (in-plane), axial loads (centrifugal), and torsional loads (pitch). Such dynamic loads subject the rotor blade to varying degrees of stresses and strains.
In addition to the operational loads to which helicopter rotor blades are subjected, the rotor blades are also subjected to a wide variety of environmental conditions during helicopter flight operations. Rotor blades that are capable of maintenance-free operation for up to several thousand hours in a non-abrasive environment may be negatively affected when operated in an environment that includes abrasive particulate matter, e.g., sand. Experience has shown that rain and/or sand particles that impinge upon the leading edges of rotating rotor blades may have an adverse effect thereon by causing erosion wear of the leading edges. Erosion wear is of particular concern at the outboard end of the rotor blades due the higher rotational velocities thereof (the rotational speed at a given span point of the rotor blade is directly proportional to the radial distance from the rotor hub) wherein the relative impact velocities of rain and/or sand particles are significantly higher. The erosion rate, E.sub.r, is proportional to velocity, V, according to an exponential relationship such as E.sub.r =f(V.sup.n) where n ranges from 2.25 to 2.50.
Several different techniques have been explored to increase the erosion resistance of the leading edges of helicopter rotor blades. One technique used by the Sikorsky Aircraft Division of United Technologies Corporation has been the incorporation of ductile metal leading edge caps as an integral part of the rotor blade. More specifically, nickel leading edge caps are net shaped to a leading edge configuration by electroforming and adhesively bonded to the outboard end of the substrate, e.g., composite, comprising the rotor blade. Nickel leading edge caps provide good wear resistance protection against rain drop impacts encountered during helicopter flight operations.
However, as experience in Desert Storm revealed, nickel leading edge caps experienced undesired erosion wear when subjected to operations in a sand particle environment such as a desert. Not only were the nickel leading edge caps subjected to erosion wear as a result of forward flight through sand storms, but nickel leading edge caps were also subjected to erosion wear as a result of hover operations, e.g., take-offs, landings, due to particulate sand motion caused by rotor blade vortices, i.e., downwash. Sand erosion wear of the nickel leading edge caps eventually lead to the need to replace such caps to maintain the desirable flight characteristics of the rotor blades. However, inasmuch as the nickel leading edge caps comprise an integral part of the rotor blade, i.e., such caps are adhesively bonded to the composite infrastructure, replacement of the eroded nickel edge caps is not a "field level" repair. Rather, eroded rotor blades are removed from the helicopter and the nickel leading edge caps thereof replaced at a "depot level" maintenance facility. This can result in undesired downtime for the affected helicopter.
To reduce the erosion wear of the nickel leading edge caps of rotor blades due to sand particle impacts, a sacrificial material may be applied to the leading edges of rotor blades. Typically, the sacrificial material is an elastomeric material such as polyurethane. While an elastomeric sacrificial coating does not provide the erosion resistance characteristics of a ductile metal such as nickel, i.e., erosion occurs at a higher rate, an elastomeric sacrificial coating does provide some advantages. The elastomeric sacrificial coating may be applied to the leading edges of rotor blades as a tape. As such, a worn elastomeric sacrificial coating may be routinely replaced at the field level by stripping off the worn tape and replacing it with new tape, thereby significantly reducing helicopter downtime. Further, the cost of elastomeric sacrificial tape coatings is significantly less than nickel leading edge caps. Moreover, erosion wear effects on elastomeric sacrificial tapes are readily observable through visual inspection.
While the use of elastomeric sacrificial tape in conjunction with rotor blades having nickel leading edge caps significantly extends the useful life of the nickel leading edge caps with respect to the effects of sand particle erosion, the maintenance cycle time for replacement of worn elastomeric sacrificial tape makes the use of such tape a less than optimal solution to sand particle erosion. Moreover, an adverse downside was discovered when rotor blades having the leading edges thereof coated with elastomeric sacrificial tape were subjected to rain particle impacts after a minimal exposure to sand particle erosion. Under such circumstances, the useful erosion protection lifetime of elastomeric sacrificial tape was significantly reduced.
A need exists to provide enhanced erosion wear protection for the leading edges of rotor blades, both main and tail rotor. Such erosion wear protection should be effective in both sand and water particulate environments, i.e., significantly reduced erosion wear rate or high erosion resistance. Such erosion wear protection should be able to accommodate the stressed environment of a helicopter rotor blade, i.e., the torsional, flapwise, and chordwise bending loads experienced by a rotor blade during flight operations.